Twenty prisoners died and 12 others were injured on Saturday in a prison on the Mexican-US border town of Matamoros, after a dispute between two inmates turned into a melee that lasted almost three hours.

President criticises opposition

Meanwhile, Felipe Calderon, the Mexican president, said on Sunday that politicians in the main opposition party may consider deals with criminals, opening an inflammatory new front in the nation's presidential election campaign.

Calderon's blunt remarks about the centrist Institutional Revolutionary Party (PRI), which is favored to win the July 1, 2012 election, are unusual in a country where the president is expected to stay largely aloof from party politics.

Centering on the policy that has dominated his presidency - an aggressive army-led crackdown on drug cartels - his comments risk polarizing opinion on how to restore stability to Mexico, where the drug war has killed 44,000 in five years.

Leading members of Calderon's conservative National Action Party (PAN), other PRI opponents and political analysts have accused the once-dominant party of making secret deals with drug cartels in the past to keep the peace in Mexico.

In a weekend New York Times interview published a day after he said a state governed by the PRI had been left in the hands of a drug gang, Calderon was asked whether the opposition party might pursue a corrupt relationship with organized crime.

"There are many in the PRI who think the deals of the past would work now. I don't see what deal could be done, but that is the mentality many of them have," Calderon, whom the law prevents from seeking a second six-year term, said.

Analysts say Calderon is bitterly opposed to the PRI, which dominated Mexico for seven decades until the PAN won the presidency in 2000 under its candidate Vicente Fox.

The tide of drug war killings has eroded support for the PAN, and the PRI's main hopeful, Enrique Pena Nieto, is said to have around twice the support of his nearest rival.